The present invention relates to methods and systems for protecting motor/compressors in refrigeration systems, including air conditioners and heat pumps, which protection methods and systems avoid the need for expensive sensors and which are capable of functioning in a variety of refrigeration system models. In this regard, the protection methods and systems of the present invention may be termed "generic" in that a single system is capable of serving a large number of different models, of widely differing capacities.
The present invention is particularly concerned with refrigeration systems of the type employed in air conditioners and heat pumps for cooling and heating living spaces. Such units are available in a wide variety of physical configurations and capacities, including small room air conditioners; self-contained reversible heat pump systems which somewhat resemble room air conditioners, but which provide both heating and cooling; central air conditioning systems which employ an indoor evaporator and a separate outdoor compressor/condenser combination; and similarly-configured heat pump systems which provide both heating and cooling by means of a reversible refrigeration system.
Such refrigeration systems, while apparently simple to control, in fact require fairly sophisticated control systems if proper operation and protection from damage under a wide variety of operating conditions, often adverse, are to be achieved.
A basic form of protection for a refrigerant motor/compressor is overload protection. Such protection is typically provided by a thermal or overcurrent sensor. In addition to overload protection, the early detection of loss of refrigerant is highly desirable, particularly in central systems which have several physically-separated components interconnected by hermetic piping having a number of joints subject to leakage. Leaks not only allow refrigerant to escape; they also allow the ingress of air and moisture. Such eventually oxidizes oil in the system, resulting in the formation of tar-like substances which damage the compressor and block restriction devices. Thus, the ingress of air and moisture into a refrigeration system is very damaging if the system is allowed to continue to run. Resulting system contamination gives rise to continued problems even after repair of the leak and refrigerant (e.g. freon) recharge has been accomplished.
Typical prior art control systems for protecting refrigeration systems employ a number of sensors so that the control system is provided with sufficient information upon which to base control decisions. For detecting leaks, a pressure sensor is typically provided on the compressor suction line. This is a relatively costly solution.
Another adverse condition is a simple high load condition, which can result when power line voltage is excessively low (a so-called "brown out" condition), or when operating under extreme ambient temperature conditions. Thus, on an extremely hot day, an air conditioning system may be subjected to both high load and low voltage. This tends to make the motor inefficient, which leads to overheating. Under such operating conditions, it is desirable to de-energize the compressor before damage results, and then allow operation to resume after a cooling-off interval.
By way of more specific example, various motor and compressor protection systems are disclosed in the following U.S. Patents: Anderson et al U.S. Pat. No. 4,038,061; Godfrey U.S. Pat. No. 4,079,432; Newell U.S. Pat. No. 4,253,130; and Genheimer et al U.S. Pat. No. 4,286,303. Of these, Anderson and Newell disclose relatively comprehensive systems for protecting air conditioners and heat pumps, and employ a variety of current and temperature sensors. Godfrey and Genheimer et al disclose motor protection systems in general which include the function of allowing a motor to attempt a restart following an overload, but only for a limited number of times.
Another approach to motor protection, particularly for a refrigeration system compressor motor, is disclosed in commonly-assigned Pohl U.S. Pat. No. 4,196,462. As disclosed in that patent, a single-phase AC induction motor of the type employing a capacitor-run winding can be protected from overload (including locked-rotor) and overspeed conditions by monitoring the voltage across the capacitor-run winding. Under heavy loading conditions, the winding voltage decreases. This can be sensed, and used to initiate appropriate protection measures, such as a timed cooling-off interval.
From the foregoing brief background, it will be appreciated that prior art protection and control systems not only require a relatively large number of diverse sensors, but also must be particularly adjusted to the size of the unit involved. Thus, overcurrent protection sized for a small air conditioner would be entirely inappropriate for a large one. By way of example, a typical product line may have from twenty to thirty different models, each requiring a customized control system.
While not prior art with respect to the present invention, it may be noted that related protection systems and methods are disclosed and claimed in commonly-assigned application Ser. No. 778,076, filed Sept. 20, 1986, by Walter J. Pohl and entitled "Self-Calibrating Control Methods and Systems for Refrigeration Systems" and now U.S. Pat. No. 4,653,285. Very briefly, the systems described in application Ser. No. 778,076 sense loading on the compressor motor, preferably by sensing the voltage across the capacitor-run winding of an AC induction motor and normalizing with respect to line voltage. A self-calibrating protection capability is implemented by utilizing the changing load as a function of time on the compressor motor during both normal and abnormal operation of a refrigeration system. More particularly, a reference value of compressor motor loading is determined and stored shortly after the start of each compressor ON cycle by allowing a stabilization interval (typically thirty seconds) to elapse, and then sensing loading and storing the sensed loading as the reference value to be used for the remainder of that particular ON cycle. In the preferred forms, it is the ratio of capacitor-run winding voltage to line voltage which is sensed and stored as a reference ratio. Thereafter, during each particular ON cycle, in order to recognize high load conditions, prevailing compressor loading is at least periodically sensed and compared to the stored reference. If the thus-sensed motor loading has increased above a high-load threshold, then a high load condition is recognized, and the compressor motor is de-energized for a timed cooling off interval. In the preferred forms, it is then-prevailing ratio of capacitor-run winding voltage to line voltage which is sensed and compared to the stored reference ratio. The compressor motor is de-energized if the then-prevailing ratio falls below a high-loaded threshold ratio established as a predetermined fraction of the reference ratio, typically 0.8 times the reference ratio.
The approach disclosed in Ser. No. 778,076 can be made self-calibrating, and compressor motor protection afforded regardless of the size of the motor, since the motor control system establishes its own reference based on the characteristics of the particular motor.
The systems and methods of Ser. No. 778,076 do not, however, provide protection against loss of refrigerant conditions, such as result from a leak.
More particularly, the power-up self-calibration technique of the above-referenced Ser. No. 778,076 cannot reliably be used to detect slow leak conditions resulting in a gradual decrease in motor/compressor load. The reason is that a slow leak of refrigerant can take effect gradually over a period of months. Each time the motor/compressor is turned ON, the self-calibration reference is shifted upward.